The present invention relates to methods and apparatus for logging earth formations penetrated by a well borehole, and more particularly to improved methods and apparatus for determining formation and borehole characteristics by the analysis of gamma rays entering the borehole from naturally occurring formation radiation. The invention thus has particular relevance to the determination of the absolute elemental abundances of potassium (K), uranium (U), and thorium (Th) in the earth formations adjacent the borehole.
Modern well logging tools typically contain gamma-ray sondes for detecting the photons emitted by naturally radioactive potassium, uranium and thorium elements. These elements are present to at least some degree in all earth formations. In recent years, gamma ray spectroscopy of earth formations to determine the weight concentrations of these three significant radioactive elements has been made practical by the development of highly stable scintillation detectors which may be lowered into a well borehole and which produce a pulse height spectrum proportional to the gamma ray energy spectrum of gamma rays impinging upon the scintillation crystal.
The number of gamma rays detected in a liquid filled borehole, however, is less than the number emitted into the borehole at the borehole wall, due to scattering and absorption phenomena in the borehole fluid and other intervening materials, such as casing, cement, and so forth. These attenuation effects can therefore result in erroneous measurements unless corrections are applied. Only in the case of an air-filled borehole is the attenuation negligible. Unfortunately, known means for applying corrections have been less than optimum, depending in many cases upon empirical compensation methods based upon comparisons with data from standard, known borehole conditions. Many boreholes, however, display parameters and conditions differing greatly from these standards. (There will be no error in the final log, of course, if the borehole conditions, such as diameter and mud weight, happen to match the standard conditions of the tool sensitivity calibration.)
A need therefore remains for a method and apparatus which can determine and compensate for such borehole attenuation effects upon the gamma radiations, which can accurately do so under widely varying borehole conditions, and which is versatile, uncomplicated, and readily suited to use in virtually all natural gamma ray spectral borehole logging applications.